Quantifying community assembly processes and identifying features that impose them

Abstract

Spatial turnover in the composition of biological communities is governed by (ecological) Drift, Selection and Dispersal. Commonly applied statistical tools cannot quantitatively estimate these processes, nor identify abiotic features that impose these processes. For interrogation of subsurface microbial communities distributed across two geologically distinct formations of the unconfined aquifer underlying the Hanford Site in southeastern Washington State, we developed an analytical framework that advances ecological understanding in two primary ways. First, we quantitatively estimate influences of Drift, Selection and Dispersal. Second, ecological patterns are used to characterize measured and unmeasured abiotic variables that impose Selection or that result in low levels of Dispersal. We find that (i) Drift alone consistently governs B25% of spatial turnover in community composition; (ii) in deeper, finer-grained sediments, Selection is strong (governing B60% of turnover), being imposed by an unmeasured but spatially structured environmental variable; (iii) in shallower, coarser-grained sediments, Selection is weaker (governing B30% of turnover), being imposed by vertically and horizontally structured hydrological factors; (iv) low levels of Dispersal can govern nearly 30% of turnover and be caused primarily by spatial isolation resulting from limited exchange between finer and coarser-grain sediments; and (v) highly permeable sediments are associated with high levels of Dispersal that homogenize community composition and govern over 20% of turnover. We further show that our framework provides inferences that cannot be achieved using preexisting approaches, and suggest that their broad application will facilitate a unified understanding of microbial communities.

Effects of dispersal and selection on stochastic assembly in microbial communities

“…On the other hand, mass effects can homogenize local communities, decreasing between-group distance and making them more similar to one another and to the regional species pool (Leibold et al, 2004;Stegen et al, 2013). We did find evidence for this type of mass effect: betweengroup distance decreased at high dispersal rates.…”

“…In contrast, dispersal can be deterministic-when certain species are better dispersers than others-or stochastic, occurring through passive processes like wind (Nemergut et al, 2013;Lowe and McPeek, 2014). Low or limited dispersal can also introduce stochasticity in microbial communities (Martiny et al, 2006;Bell, 2010;Lindström and Östman, 2011), potentially through increased drift (Hanson et al, 2012;Stegen et al, 2013), whereas high rates of dispersal can induce mass effects. Through mass effects, high dispersal rates can swamp selection, making microbial communities more similar to a regional species pool than expected by chance and less predictable from environmental variables (Leibold et al, 2004).…”

Quantifying the relative roles of selective and neutral processes in defining eukaryotic microbial communities

“…Studies examining gastrointestinal and soil subsurface bacterial communities show the primacy of selection, but also indicate that neutral processes have a secondary role (Jeraldo et al, 2012;Stegen et al, 2012Stegen et al, , 2013Wang et al, 2013). In contrast bacterial communities in wastewater plants appear primarily defined by neutral community assemblage (Ofit¸eru et al, 2010), and neutral effects primarily influence desert photosynthetic bacterial assemblages (Caruso et al, 2011).…”

“…Thus, studies that sample across reasonable distances tend also to sample different environments, confounding one's ability to determine whether neutral or selective processes largely define any observed differences in communities (Bell, 2001;Chase and Myers, 2011;Ranjard et al, 2013). So far statistical treatments have been developed to attempt to overcome this problem separating these effects (Dumbrell et al, 2010;Chase and Myers 2011;Stegen et al, 2012Stegen et al, , 2013.…”

Stochastic distribution of small soil eukaryotes resulting from high dispersal and drift in a local environment

“…DDS constitutes a general pattern across different groups of organisms, and it is widely employed to disentangle the relative importance of neutral and niche processes in maintaining biodiversity (Condit et al, 2002;Cottenie, 2005;Hanson et al, 2012;Stegen et al, 2013). DDS results from the joint effects of environmental selection and dispersal limitation coupled with ecological (or evolutionary) drift.…”